1. Field of the Invention
The present invention relates to an image reading apparatus. More particularly, the present invention relates to a contact-type line image scanner for optically reading a document paper sheet in full color. The present invention also relates to an image sensor chip which may be advantageously incorporated in such an image scanner.
2. Description of the Related Art
In general, a full-color line image scanner comprises a light source for irradiating a document paper sheet with white light, apluralityof red light receiving elements for detecting a red light component of the white light reflected on the document sheet, a plurality of green light receiving elements for detecting a green light component of the reflected white light, and a plurality of blue light receiving elements for detecting a blue light component of the reflected white light. Each of the light receiving elements converts the detected amount of light into a corresponding electric signal.
FIG. 20 of the accompanying drawings illustrates a typical arrangement of light receiving elements in a full-color line image scanner. As shown, a plurality of red light receiving elements 80R, a plurality of green light receiving elements 80G, and a plurality of blue light receiving elements 80B are regularly arranged in a common single row. Specifically, groups of light receiving elements are arranged in a common row in such a manner that each group includes a red light receiving element 80R, a green light receiving element 80G and a blue light receiving element 80B. Each group of three different light receiving elements 80R, 80G, 80B, which corresponds to a single pixel or picture element, has a primary scanning direction dimension S2 and a secondary scanning direction dimension S3, whereas each light receiving element has a primary scanning direction dimension S1. Further, each of the light receiving elements 80R, 80G, 80B is covered by a correspondingly colored filter which allows selective passage of a relevant color component of the white light.
Since the three kinds of light receiving elements detect three different color components of the white light at the same time, the image scanner described above is capable of full-color image reading at high speed. However, the prior art image scanner has been found disadvantageous in the following points.
First, since each group of three different light receiving elements 80R, 80G, 80B corresponds to a single picture element, the primary scanning direction dimension S2 needs to be set equal to or less than the primary scanning direction dimension of the single picture element. Therefore, the primary scanning direction dimension S1 of each light receiving element must be set less than 1/3 of the primary scanning direction dimension of the single picture element. As a result, the light receiving area of each light receiving element inevitably becomes small, thus leading to a poor reading quality (i.e., a low electrical output). Such a problem is particularly remarkable when the pitch between the respective light receiving elements 80R, 80G, 80B is set small to increase the reading resolution.
Secondly, since the three different light receiving elements 80R, 80G, 80B in each group (corresponding to a single picture element) deviates positionally in the primary scanning direction, there may be a color deviation when the three light receiving elements in the same group read the same picture element of the document sheet.
In the third place, since the three light receiving elements 80R, 80G, 80B in each group are dedicated to different colors, they need to be separately covered by different filters. Therefore, there is a need for separately attaching as many filters as the light receiving elements, consequently resulting in a cost increase.
The above-described problems may be eliminated by providing three different light sources for emitting red light, green light and blue light toward a document sheet with a time lag. In this case, a single kind of light receiving elements in a single row is caused to successively detect the reflected different color light components.
However, the latter solution is also disadvantageous in that the image reading speed becomes low due to the need for actuating the different light sources with a time lag. Further, since each light receiving element (which is normally provided by a phototransistor) is successively caused to detect different colors, unexpected mixture of two or more colors may occur due to incomplete discharge of a charge of the phototransistor. More specifically, as shown in FIG. 21, a phototransistor constituting each light receiving element retains a certain amount of previous charge (represented as "charge remainder ratio") even after discharging for another light detection, so that two successive detection for two different colors may result in unexpected color mixture.